Large-scale structure of galaxies in the Ophiuchus region

The large-scale structure around the Ophiuchus cluster of galaxies in the v icinity of the Galactic Centre (l = 0.degrees 5, b = 9.degrees 5, cz = 8500 km s(-1)) is investigated on the basis of a galaxy survey and spectroscopi c observations made for a 12 degrees x 17 degrees area. The galaxy survey w as performed using six ESO/SERC Sky Survey Atlas films, and 4021 galaxies w ere detected in total. Recession velocities were newly obtained for 179 gal axies to make the total number of galaxies in the survey area with known ve locities 219. In the distribution of bright galaxies, we identified seven new clumps of g alaxies. Comparing the surface number density of bright galaxies with the G alactic extinction, which is estimated from the 100-mu m flux density in th e IRAS Sky Survey Atlas, we demonstrate that the seven clumps are not spuri ous as a result of the inhomogeneity of the Galactic extinction. Among the seven clumps, two are found to be clusters and four to be groups on the bas is of the histogram of recession velocities and the number of member galaxi es. The Ophiuchus cluster, two newly identified clusters, and four groups a re all concentrated at 9000 km s(-1). Field galaxies are also distributed c entred at 8500 km s(-1). Hence field galaxies occupy a common three-dimensi onal region with galaxies in the clusters and groups, and altogether they f orm a large-scale structure of supercluster size. As opposed to the overden sity in the supercluster region, the mean number density of galaxies in the velocity range 0-5000 km s(-1) is only 25 per cent of the mean number dens ity of the Universe, comparable with the density of the well-known Bootes v oid. Hence this nearby three-dimensional region in Ophiuchus is a void of g alaxies also.